Comparative estimation of nitrogen in urea and its derivative products using TKN, CHNS and hand-held refractometer

The utilization of handheld refractometer based detectors has gained significant attention because of their small size, low-cost, portable, no need for any skilled person, and real-time detection. In this device, a few drops of liquid suspension containing urea can be placed on the prism, and therefore, the reading is displayed on the screen within 3 s. This handheld instrument works on the principal of refractive index. When a liquid sample is placed on the prism surface then the light is transmitted via the solution, while some of the light is reflected and detected by photodiodes. As a consequence, a shadow line is created whose position is closely related to the refractive index of the solution. Therefore, the refractive index or another unit of measure as per refractive index is correlated by internal software after the determination of the position of the shadow lines by using the instrument. Thus, the final concentration of liquid can be viewed on the display of the refractometer. This device is easy to calibrate by simply mounting a few drops of liquid standard or distilled water on the surface of the prism.

In order to determine whether refractometer as a substitute method to TKN and CHNS for N% estimation for conventional urea as well as nano urea fertilizers, all three techniques were systemically compared in terms of linearity, and on the basis of other performance such as cost, sample throughput, environmental acceptance, and automation capabilities. The calculation of N% in different concentrations of urea and nano urea was analyzed by TKN, CHNS, and refractometer and then compared. Table 1 shows the descriptive statistics of data utilized to compare the N% analysis by using different methods (where duplicate was taken for each samples; n = 2).

Table 1 Descriptive statistics of data utilized to compare TKN, CHNS, and Refractometer based methods for nitrogen analysis (N%).

Moreover, their comparison has been evaluated in terms of correlations of the results obtained by a different technique. Figure 1a – d shows the linear fitting between the estimated nitrogen (%) and various concentrations of urea ranging from 0.25 to 10% for TKN, CHNS, refractometer, and theoretical value of expected N%.

Figure 1
figure 1

Linear fitting between nitrogen (%) and different concentration of urea from 0.25 to 10% for (a) TKN, (bCHNS (c) refractometer, and (d) theoretical N%. (Both CHNS and refractometer have shown the R2 value is close to the theoretical value for urea analysis).

Interestingly, the refractometer was capable of detecting the lower limit of urea at 0.25% (0.11% N) and displayed 0.092% N. Moreover, it was observed that the N% detected by TKN exhibited the straight line with the lowest R2 (0.98879) value as compared to other techniques. Whereas, both CHNS and refractometer have detected the different concentrations of urea as their R2 value is close to the theoretical value of N%. Furthermore, the estimation of N% in nano urea was performed by using all techniques to evaluate the instrument performance in terms of linearity. A linear fitting between the measured N (%) and different concentrations of nano urea ranging from 1 to 10% for TKN, CHNS, and refractometer can be seen in Fig. 2a – d.

Figure 2
figure 2

Linear fitting between nitrogen (%) and different concentration of nano urea (1–10%) for (a) TKN, (bCHNS (c) refractometer, and (d) theoretical N%. (Refractometer showed R2 value is close to the theoretical value for the analysis of nano urea).

These results indicated that the refractometer-based device showed R2 = 0.99935 with an intercept of – 0.04667 ± 0.02455 and a slope of 0.46667 ± 0.00396. It can be concluded that the linear fitting for the estimation of N% in nano urea by refractometer is more closed to the theoretical value of N% as compared to other methods. Furthermore, in view of the importance of the DEF solution in diesel engines for air pollution prevention, higher concentrations of urea up to 40% were utilized for the analysis. A linear fitting between nitrogen (%) and different concentration of urea ranging from 0.25 to 40% for TKN, CHNS, refractometer, and the theoretical N% have been displayed in Fig. 3a – d. On these concentrations, the refractometer-based device produced outstanding results in terms of R2 (0.99918) in comparison with other techniques, which indicates that the refractometer can also be used for the analysis of DEF solution.

Figure 3
figure 3

Linear fitting between nitrogen (%) and different concentration of urea ranging from 0.25 to 40% (including DEF at 32.5% urea) for (a) TKN, (bCHNS (c) refractometer, and (d) theoretical N% (Refractometer showed R2value is close to the theoretical value for urea analysis).

Table 2 displays the different techniques for the measurement of urea and nano urea sample containing concentration up to 10% and their respective values ​​such as R2intercept and slope extracted via linear fitting.

Table 2 Different analysis methods for various concentrations (up to 10%) of urea and nano urea and their R2intercept, and slope values ​​extracted by linear fitting.

In order to further evaluate, the measured N% of different concentrations of urea and nano urea (1, 5, and 10%) were extracted by all methods and compared as shown in Fig. 4a, b.

Figure 4
figure 4

N% detection by using different techniques and their comparison with the theoretical value of different concentration of (a) urea and, (b) nano urea (c) estimation of N% in DEF solution containing 32.5% urea by using different methods (refractometer, CHNS, and TKN) and compared with theoretical value of N%. (Refractometer based device showed less deviation ~ 1.53% with theoretical value as compared to other techniques in case of DEF solution).

It was observed that N% estimation by TKN is inconsistent with the theoretical value of N%. The reason is this technique only detects nitrogen from ammonium as well as organic constituents such as amino acids, nucleic acids, and proteins in the sample. However, it is not possible for the measurement of other forms of nitrogen present in nitrite and nitrate using TKN technique8. Moreover, refractometer-based results are more closed to that of the theoretical value of N% as compared to other techniques as shown in Fig. 4. Figure 4c displays the analysis of N% in DEF solution by utilizing different methods such as CHNS, TKN, and refractometer and then compared their results with the theoretical value of N%. In this analysis, TKN, CHNS, and refractometer based device showed 15.27, 14.01, and 14.72% N-content in DEF, respectively. It was observed that the N% was deviated by + 6.29% in TKN, – 2.14% in CHNS, and – 1.53% in refractometer with the theoretical value of N% in DEF. Therefore, the refractometer based device exhibited the more close value to theoretical N% in case of DEF.

Other performances

Measurement time and automation: When analyzing a large number of samples in laboratories, then these characteristics have a significant concern. For example, TKN can analyze only 8 samples (including two replicates of each sample and two blank) using one digestion block for 8 tubes and one distillation-titration unit within 4 h. In the case of CHNS, around 13–17 analyses can be executed in the same period. In contrast, a refractometer can analyze about 170–180 samples (including sampling, testing, and washing the prism surface) in 4 h. Thus, this device enables the fast detection of N% for urea samples.

Dealing with the automation capabilities, TKN has some manual steps (for example, insertion of reagents in digestion tube, dilution of chemicals after digestion and positioning the digestion tube in distillation system). Moreover, samples analysis by CHNS has some advancement in terms of samples insertion via autosampler. On the other hand, one can easily monitor the results by pressing the button on a refractometer after placing a few drops of liquid samples.

Environmental and safety perspective: The utilization of hazardous acids (sulfuric acid, sodium hydroxide) and catalyst-based heavy metals are a great concern while using the TKN. Moreover, there are a small number of uses of heavy metals during the sample analysis by CHNS. On the contrary, refractometer does not require any hazardous chemicals and toxic elements during the sample analysis. Surprisingly, DI water is only used for the washing of the prism surface after the measurement.

Cost: Both instruments (TKN and CHNS) are expensive. The price of sample analysis includes their fixed (instrument cost), variable (glassware, standard chemicals, other chemicals, power, water consumption, and maintenance cost), and labor cost of a technician. On the other hand, a hand-held refractometer is quite cheap as compared to both instruments. The device operation is simple and no other cost is involved. For all these reasons, hand-held refractometer-based urea detection could be a possible technique in the field of fertilizer industries.

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